Metastatic castration-refractory prostate cancer (CR-PC) patients have poor prognosis with median survival of approximately 14 months. 10-20% of all prostate cancer patients develop metastatic CR-PC within 5 years of initial diagnosis, and therefore, improved therapeutic strategies are needed. Oligometastasis describes a clinical state where metastases are limited in number, and metastatectomies for various oligometastatic solid tumors can lead to long-term survival. Stereotactic Ablative Radiotherapy (SABR) is a noninvasive therapy with local control (LC) similar to surgery. In our experience, oligometastatic prostate cancer patients (65% were CR-PC) experienced 100% LC at 6 months with SABR with corresponding decline in their PSA. 11C-Choline PET/CT was FDA-approved in recurrent metastatic prostate cancer patients, and prior restaging studies have validated the sensitivity, specificity, positive predictive value, and negative predictive value of Choline PET at 85-100%, 76-96%, 76-91%, and 81-100%, respectively. Therefore, 11C- Choline PET/CT may help to identify true oligometastatic CR-PC patients. Objectives/Hypothesis: The combination of 11C-Choline PET/CT and SABR may improve true oligometastatic (CR-PC) patient selection, alter their clinical management (i.e., from noncurative to curative intent with the addition of SABR), and impact natural disease progression (improve LC, PSA progression-free survival and overall survival [OS]). SABR may also induce anti-prostate cancer immunity, which could be amplified into long-term protective immunity with immune-regulation inhibitors (e.g., ipilimumab, anti-PD1/PD-L1). Specific Aims/Study Design: We propose a prospective single arm phase II clinical trial to assess the role of 11C-Choline PET/CT and SABR in metastatic CR-PC patients with the goal of improving clinical outcomes (Aim 1). We also propose a translational study in Aim 2 to explore the induction anti-prostate cancer immunity elicited by SABR treatments. The confirmation of the ability of SABR to induce anti-prostate cancer immunity would provide a robust rationale to combine SABR and immunomodulating agents (i.e., checkpoint inhibitors) in widespread castration- and chemotherapy- resistant metastatic prostate cancer. The proposed biomarker (CD11ahighPD-1high CD8+ T cells) may improve identification of potential responders to the combined SABR and anti-PD1 therapy. Impact: The incorporation of 11C-Choline PET/CT in the selection of oligometastatic CR-PC patients whose limited metastases are amenable to SABR may improve OS. If our hypothesis is proven successful, there would be a paradigm shift in the clinical management of these patients (i.e., from noncurative to curative intent). The in situ induction of anti-prostate immunity by SABR, if confirmed, would form a robust rationale for the combination of SABR and immunomodulating agents in future clinical trials.